Hot melt adhesives have gained acceptance in both industrial and domestic environments. Hot melt adhesives generally comprised a thermoplastic which was substantially solid at room temperature and which melted when heated. Various thermoplastic adhesive have been developed for different purposes such as gluing calking and sealing. Generally, the thermoplastics become molten and tacky at temperatures ranging from 370.degree. F. to 400.degree. F. (188.degree. C. to 204.degree. C.). When in a molten state, the hot melt adhesives adhered to most surfaces. Among the advantages of hot melt adhesives were that they rapidly set, provided a waterproof bond and were not affected by most solvents.
Hot melt adhesives have been used to bond such surfaces as wood, plastics, foam, cardboard, fabrics, ceramics and paper. They have also been employed as a sealing or caulking material. In addition, they were electronically insulative and have been employed to seal and insulate electronic components.
Glue guns have been employed to melt and extrude hot melt adhesives. For glue gun applications, the thermoplastic hot melts have been formed into cylindrical rods, commonly referred to as glue sticks, which were available in lengths of 2 to 10 inches (5 to 26 cm). The glue guns included a glue feed path having an electric heating element with a cylindrical bore adapted to receive successive glue sticks at its upstream end and an extrusion nozzle at its downstream end.
The user fed glue sticks by hand in occasional use, low cost, glue guns. The danger of accidental injury when using such glue guns, uneven advance stroke and other factors, limited the usage of such guns. Trigger advance mechanisms have been provided for forcing glue sticks into the heating elements in better quality glue guns. Previous trigger advance mechanisms were relatively complex and included levers and claws for engaging the outside of the glue stick.
In the advance mechanism employed, for example, in the No. 216 Bostic Thermogrip glue gun, a trigger was connected to a double lever which engaged a feeder slide. Other trigger advance mechanisms included three or more spring loaded levers with the last lever having a claw which directly engaged the outside of a glue stick. Generally, the trigger advance mechanisms of the past included a relatively high number of complex components resulting in high assembly costs.
An additional disadvantage of prior glue gun trigger advance systems was that due to the nature of the glue stick gripping mechanisms which engaged the outer surface of the glue sticks, the engaging claw or lever had a tendency to impale the glue sticks. This tendency was, in part, due to the softening of the glue stick from the heat generated at the downstream heating element and/or softness of the glue stick itself at ambient temperatures. Penetration of the claw or lever into a glue stick did not result in glue stick advancement.
As a result of the impalement of the claw or lever in the glue sticks, the lever mechanisms became imbedded in the glue sticks which impeded the return of the levers to their original positions when the trigger was released. Usually, the lever mechanisms included a return spring for retracting the levers upon release of the trigger. Once the levers or claws became embedded into a glue stick, repair involved difficult procedures including possible disassembly of the gun.
In order to reduce the tendency of prior levers or claws becoming impaled in a glue stick, many glue guns included apertures through the glue gun casings. The apertures provided not only for cooling, but in addition, access to the lever mechanism for the purpose of freeing levers or claws which were lodged in the glue stick.
On the other hand, in instances wherein the glue stick surface was relatively hard, an imbalance of lever force components resulted in slippage without positive advancement of the glue stick toward the heating element.
A further problem which has been encountered with prior glue gun feed mechanisms has been the relatively short glue stick advance for a trigger stroke. This was principally due to the operation of the force multiplying lever arrangements. In some instances, a glue stick advance as short as 3/8 to 5/8 inch (10 to 15 mm) resulted from a 21/2 inch (65 mm) trigger stoke. Such short glue stick advance resulted in relatively short extruded sealing lines for each trigger stroke. In sealing applications, a continuous uninterrupted sealing line was preferred. With relatively short glue stick advances, short sealing lines resulted and a uniform sealing line greater than, for example, 40 mm in length was often impossible.